The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith and incorporated by reference, employs the oscillometric methodology. In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. Then, the cuff pressure is reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant arterial pulse signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "oscillation complexes" or just simply "oscillations"). The oscillation complexes typically have amplitudes which are typically about one percent that of the arterial pulse signals. After suitable filtering to reject the DC component and to provide amplification by a scale factor, peak pulse amplitudes (PPA) above a given base-line are measured and stored. As the decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These amplitudes form an oscillometric envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure ("MAP"). Systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use, the blood pressure cuff is placed on the patient and the operator usually sets a time interval, typically from 1 to 90 minutes, at which blood pressure measurements are to be made. The noninvasive blood pressure ("NIBP") monitor automatically starts a blood pressure determination at the end of the set time interval.
FIG. 1 illustrates a simplified version of the oscillometric blood pressure monitor described in the aforementioned Ramsey patents. In FIG. 1, the arm 100 of a human subject is shown wearing a conventional flexible inflatable and deflatable cuff 101 for occluding the brachial artery when fully inflated. As the cuff 101 is deflated using deflate valve 102 having exhaust 103, the arterial occlusion is gradually relieved. The deflation of cuff 101 via deflate valve 102 is controlled by microprocessor 107 via control line 108.
A pressure transducer 104 is coupled by a duct 105 to the cuff 101 for sensing the pressure therein. In accordance with conventional oscillometric techniques, pressure oscillations in the artery are sensed by changes in the counter-pressure of the cuff 101, and these pressure oscillations are converted into an electrical signal by transducer 104 and coupled over path 106 to microprocessor 107 for processing. In addition, a source of pressurized air 109 is connected via a duct 110 through an inflate valve 111 and a duct 112 to the pressure cuff 101. The inflate valve 111 is electrically controlled through a connection 113 from the microprocessor 107. Also, the deflate valve 102 is connected by duct 114 via a branch connection 115 with the duct 112 leading to cuff 101.
Microprocessor 107 processes the signals from pressure transducer 104 to produce blood pressure data and to reject artifact data as described in the afore-mentioned Ramsey '029 and '034 patents. However, the blood pressure also can be determined in accordance with the teachings of Medero et al. in U.S. Pat. No. 4,543,962, of Medero in U.S. Pat. No. 4,546,775, of Hood, Jr. et al. in U.S. Pat. No. 4,461,266, of Ramsey, III et al. in U.S. Pat. No. 4,638,810, of Ramsey, III et al. in U.S. Pat. No. 4,754,761, of Ramsey, III et al. in U.S. Pat. No. 5,170,795, and of Ramsey, III et al. in U.S. Pat. No. 5,052,397, all of which are commonly assigned herewith and the disclosures of which are hereby incorporated by reference. Any of these known techniques are used to determine the quality of the oscillation complexes received at each level so that the blood pressure determination is made using actual blood pressure data and not artifacts.
During operation of the apparatus illustrated in FIG. 1, it is assumed that air under pressure to about 8-10 p.s.i. is available in the source of pressurized air 109. When it is desired to initiate a determination of blood pressure, the microprocessor 107 furnishes a signal over path 113 to open the inflate valve 111. The deflate valve 102 is closed. Air from the source 109 is communicated through inflate valve 111 and duct 112 to inflate the cuff 101 to a desired level, preferably above the estimated systolic pressure of the patient. Microprocessor 107 responds to a signal on path 106 from the pressure transducer 104, which is indicative of the instantaneous pressure in the cuff 101, to interrupt the inflation of the cuff 101 when the pressure in the cuff 101 reaches a predetermined value above the estimated systolic pressure of the patient. Such interruption is accomplished by sending a signal over path 113 instructing inflate valve 111 to close. Once inflate valve 111 has been closed, the blood pressure measurement can be obtained by commencing a deflate routine.
Actual measurement of the blood pressure under the control of the microprocessor 107 and the deflate valve 102 and as sensed by pressure transducer 104 can be accomplished in any suitable manner such as that disclosed in the aforementioned patents. At the completion of each measurement cycle, the deflate valve 102 can be re-opened long enough to relax the cuff pressure substantially completely via exhaust 103. Thereafter, the deflate valve 102 is closed for the start of a new measurement cycle.
Accordingly, when a blood pressure measurement is desired, the inflate valve 111 is opened while the cuff pressure is supervised by pressure transducer 104 until the cuff pressure reaches the desired level. The inflate valve 111 is then closed. Thereafter, the deflate valve 102 is operated using signal 108 from microprocessor 107 and the blood pressure measurement taken.
Prior art FIG. 2 illustrates a pressure versus time graph illustrating a conventional cuff step deflation and measurement cycle for a conventional NIBP monitor. As illustrated, the cuff 101 is inflated to a pressure above the systolic pressure, and the cuff 101 is then deflated in steps of equal duration of about 8 mm Hg per step. A timeout duration d is provided at each step during which the signal processing circuitry searches for oscillation complexes in accordance with the techniques described in the afore-mentioned commonly assigned patents. At the end of timeout duration d, the cuff pressure is decremented even if no oscillation complex is detected. This process of decrementing the pressure and searching for oscillation complexes is repeated at least until MAP and/or the oscillometric envelope 200 may be determined. The entire blood pressure determination process is repeated at intervals set by the user or some other predetermined intervals.
As shown in FIG. 2, the patient's arterial blood pressure forms an oscillometric envelope 200 comprised of a set of oscillation amplitudes measured at the different cuff pressures. From oscillometric envelope 200, systolic, MAP and diastolic blood pressures are typically determined. Typically, these blood pressure determinations require that the maximum amplitude (MAP) be known. Indeed, as noted above, MAP is found at the lowest cuff pressure at which the oscillations have a maximum amplitude. From this maximum amplitude, amplitudes for the systolic and diastolic pressures are calculated using known equations and the corresponding cuff pressures are determined and displayed as the systolic and diastolic pressures. MAP is then computed and displayed from amplitudes and pressures on the low (diastolic) and high (systolic) pressure sides of the known maximum amplitude.
Unfortunately, this method for calculating MAP is somewhat sensitive to erroneous high oscillation amplitudes which are mistakenly treated as corresponding to the MAP pressure. This is so because a single point is used to define MAP and systolic pressures, and only one or two points are used to define diastolic pressure. An envelope analysis technique is desired which is less sensitive to erroneous oscillation amplitudes and can thus provide a more accurate MAP determination.
It is, accordingly, a primary object of the present invention to provide an oscillometric envelope analysis method which is less sensitive to erroneous oscillation amplitudes.
It is a further object of the present invention to weigh all points along the oscillometric envelope equally and to use all measured points in the MAP determination.
It is also an object of the present invention to provide a linear approximation technique for determining the oscillometric envelope and simplifying signal processing requirements.